Ensure non-ASCII characters are typeset correctly even if PS_CHAR_ENCODING is not 'ISOLatin1+'

[seisman]

Description of proposed changes

Non-ASCII characters may not be typeset correctly if the default PS_CHAR_ENCODING setting is not ISOLatin1+. This can happen when (1) GMT is compiled with US units as default; (2) users have a global gmt.conf file with PS_CHAR_ENCODING set to Standard+; (3) users set PS_CHAR_ENCODING to Standard+ in the middle of a script.

Taking the gallery example at https://www.pygmt.org/dev/tutorials/advanced/non_ascii_text.html as an example.

If you have a gmt.conf file with PS_CHAR_ENCODING set to Standard+ (e.g., you can create a gmt.conf file by running GMT command gmt set PS_CHAR_ENCODING=Standard+ in terminal), running the gallery example will result in the incorrect figure shown in the left column below:

PS_CHAR_ENCODING=Standard+	PS_CHAR_ENCODING=ISOLatin1+

Previously, we assumed the default PS_CHAR_ENCODING is ISOLatin1+ and didn't add --PS_CHAR_ENCODING=ISOLatin1+ if the text string contains non-ASCII characters in the ISOLatin1+ charset. This PR fixes the issue by always adding --PS_CHAR_ENCODING=<encoding> even if encoding is ISOLatin1+.

In 8c34683, the existing test test_text_nonascii is modified to show that setting PS_CHAR_ENCODING to Standard+ breaks the test (https://github.com/GenericMappingTools/pygmt/actions/runs/11800822308/job/32872646796?pr=3611). The fix is added in d9e5203.

Closes #2204.

[seisman]

Ideally, we should test the case that PS_CHAR_ENCODING is set to "Standard+" in a global "gmt.conf" file. However, it's technically difficult. Similar to the test test_gmt_compat_6_is_applied at

pygmt/pygmt/tests/test_session_management.py

Line 31 in 763dc07

def test_gmt_compat_6_is_applied(capsys):

Generally we need to:

1. Generate a gmt.conf file in the current directory
2. Kill the global session
3. Start a new session
4. Create a new figure and adding non-ASCII characters to it
5. End the session so the "gmt.conf" file won't affect other tests
6. Start a new, clean global session

But the issue is that, after starting a new global session, we can't return the Figure instance for comparing images, because mpl_image_compare needs to call Figure.savefig which no longer works after the session is destroied.

So here we only test the case that PS_CHAR_ENCODING is changed in the middle of a script.